WET Sensor User Manual v1.4
Contents
1. User Manual for the WET Sensor type WET 2 WET UM 1 4 A H Delta T Devices Ltd Notices Copyright All rights reserved Under the copyright laws this manual may not be copied in whole or in part without the written consent of Delta T Devices Ltd Under the law copying includes translation into another language Copyright 2007 Delta T Devices Ltd Acknowledgements This sensor system has been developed in co operation with Wageningen University and Research Center The Netherlands GREENHOUSE TECHNOLOGY PPO WAGENINGEN BG Naaldwijk The Netherlands PRAKTIJUKONDERZOEK Bn Bows amp OMGEVING W wh Ge Ge Pel e G E 5 grg and Saint Gobain Cultilene B V ntti lm Tilburg ro SAINT GOBAIN The Netherlands Cultil ne CULTILENE Trademarks All manufacturers trademarks are acknowledged Some names referred to are registered trademarks CE conformity The WET Sensor type WET2 conforms to EC regulations regarding electromagnetic emissions and susceptibility when used according to the instructions contained within this user manual and is CE marked by Delta T Devices Ltd Design changes Delta T Devices Ltd reserves the right to change the design and specification of its products at any time without prior notice User Manual WET UM 1 4 February 2007 Delta T Devices Ltd Tel 44 0 1638 742922 130 Low Road Burwell Fax 44 0 1638 743155 Cambridge CB25 0EJ email sales delta t co uk UK2. 15 Dielectric properties 4 5 19 22 23 24 30 33 Display 11 16 19 reading 22 23 WET Sensor User Manual v1 4 E Electrical conductance 34 conductivity 4 27 34 E Field capacity 11 12 14 26 36 Frequency 24 27 30 32 35 H HH2 4 5 6 8 9 10 11 12 13 15 16 18 19 20 32 HH2Read 9 IMAG ii 15 30 40 Insertion 4 6 7 10 22 23 Ions 13 18 29 34 Maintenance 7 Measurement Principle 5 Memory 9 Mineral soil 12 Moisture Meter 4 5 6 8 9 10 11 12 13 15 16 18 19 20 32 N Notes 21 22 23 O Options 10 11 Organic soil 12 31 Output file 13 P Permittivity 21 35 PPO 15 40 Index 41 Q Quick start 8 R Range 21 Reading accuracy 21 range 21 Readings range 7 15 16 17 27 29 35 Recalibration 20 References 33 Rooting depth 11 S Salinity 30 31 33 35 Sensor care 7 Servicing 39 Set device 8 9 11 display 11 16 19 field capacity 11 12 36 soil set up 10 12 soil type 10 12 temperature compensation 23 units 11 21 34 Software CD or disk 6 Soil 42 Index calibration 14 19 clay 19 17 22 23 25 29 36 dry 7 15 16 17 27 29 35 extract 34 magnetic 22 23 mineral 12 moisture 4 14 21 23 33 34 organic 12 31 parameter 12 17 19 sand 12 31 Soil type 10 12 Specifications 21 Store 8 10 Temperature 4 5 10 11 13 18 19 21 23 compensation 13 1
3. A Lesch S M 1990 Determining soil salinity from soil electrical conductivity using different models and estimates Soil Science Society of America Journal 54 46 54 WET Sensor User Manual v1 4 Technical Reference 33 Definitions of Terms The Electrical Conductivity EC of a material is a measure of its ability to carry an electrical current It is an intrinsic property of the material into which the electrodes are inserted Le a property which is defined at a point and does not depend on how much material is present q v density In contrast the Electrical Conductance G is a measure of the current carrying ability of an extensive sample of material and depends on the particular measurement set up particularly the length L and cross sectional area A of the measurement cell Conductivity and conductance are related by the formula eee ee A GC S m where C is the Cell Constant Electrical Conductivity is measured in Units of Siemens per meter S m We have used m mt throughout this manual because it is an SI preferred unit The following conversions apply 1mS m 0 01 mS cm 10 uS cm Pore Water Conductivity EC or o is the electrical conductivity of the water within the soil pores It is determined by the concentration of different ions within the pore water and by the temperature In contrast the Bulk Electrical Conductivity EC or op is the total electrical cond
4. m sample The Permittivity of a material characterises its response to the polarising effect of an applied electric field It is usually represented as a complex number EA je where the real part of the permittivity 4 represents the energy stored and the imaginary component represents the total energy absorption or loss Both values are frequency and temperature dependent Permittivity is commonly used as a means of measuring water content because the real permittivity of water is 80 at 20 MHz 25 C whereas the permittivity of most soil particles is typically in the range 3 to 8 Dielectric is best used as a descriptive term e g dielectric materials usually refers to insulating materials with a high relative permittivity Dielectric constant is sometimes used interchangeably for permittivity but may be more rigorously defined as the real part of the permittivity in a static electric field Saturation is the moisture content at which all the air within the pores has been replaced by water It s not a stable situation WET Sensor User Manual v1 4 Technical Reference 35 because the water will immediately start to drain through It s a property of soil type only Field Water Capacity or Field Capacity is the moisture content obtained when a saturated soil has been allowed to drain sometimes taken as 2 days later sometimes when drainage has become negligible It s a proper
5. of the WET Sensor amp the integrated circuit which enables accurate measurement of the permittivity and conductivity of the bulk soil or media e eil Fre Cultil ne AN GChAN CULTILENE Saint Gobain Cultilene B V Zeusstraat 2 5048 CA TILBURG The Netherlands Tel 31 0 13 5780057 Web site www cultilene com Sponsors of research into horticultural media applications amp suppliers of horticultural media calibrations PRAKTIJRONDERZOE KF PLANT amp OMGEWING WAGENINGEN grp Applied Plant Research Division Glasshouse Horticulture Praktijkonderzoek Plant amp Omgeving B V the former Research Station for Floriculture and Glasshouse Vegetables P O Box 8 2670 AA Naaldwijk The Netherlands Web site www ppo wur nl Developers of the WET calibrations for horticultural media Copyright 2001 Praktijkonderzoek Plant amp Omgeving B V 40 Technical Support WET Sensor User Manual v1 4 Index A Accuracy 21 ASIC 30 B Battery life 9 C Calibration file 6 9 11 19 generalised 4 12 14 15 25 recalibration 20 sensor 19 soil 14 19 soil specific 15 WET CL 12 15 21 Case 6 Compost 15 coir 15 glasswool 15 mineral wool 15 peat 15 Conductivity bulk 18 Electrical 4 27 34 ionic 18 24 27 28 29 32 pore water 4 5 10 11 12 13 17 20 23 26 34 Connection 8 9 21 37 Cultil ne 15 40 D Data erase 11 plot 11 root depth 11 sample 11 Description 4 6
6. soil deficit mmDef The HH2 User Manual contains information and advice on obtaining values of field capacity for different soils This option is only available for Custom1 Custom5 soil types The soil water content is calculated from the soil dielectric reading using an offset value bo and a scaling factor b bO can be set to a value between 25 25 but should normally be between 1 0 and 2 3 b1 b1 can be set between 0 49 9 but should normally be between 6 0 and 12 0 You can choose to display and store the Conductivity conductivity readings in either mS m 1 mS cm 1 or uS cm 1 Water Choose either vol or m3 m 3 ontent Display By default the HH2 calculates displays and stores the Water Content Pore Water or Conductivity ECp and the Temperature It can be set to display the permittivity eb and W ET ECb a GE bulk soil conductivity ECb as well for reference or Alternatively the HH2 can be set to calculate and display the soil deficit i e the amount of water W E T mmDef required to refill a soil to its field capacity see also Root Depth and Capacity 12 Taking Readings WET Sensor User Manual v1 4 Compensation Conductivity readings both Pore Water and Bulk are affected by temperature and it is common practise to apply a correction so that they are Temperature quoted at a standard temperature You can choose to apply temperature compensation to 20 C or 25 C or to take
7. uncompensated readings None Used to set the rate at which temperature compensation will be applied The default value is Percent 2 0 but you may choose between 1 0 4 0 See Calibration section for suggested values for different ions Output File Details of the output file from the HH2 are given in HH2 user manual but for WET Sensor readings the file will look similar to this Device gt gt WET Root Depth gt gt 0 Sensor Depth gt gt 0 Soil gt gt Mineral BO gt gt 1 6 B1 gt gt 8 4 Soil Parameter 4 1 Field Capacity 0 38 EC Compensation None Time Sample Plot Device VWC ECp Tmp E b ECb Vol mS m 1 degC mS m 1 01 01 00 23 51 1A 0 27 5 166 1 27 6 159 24 01 01 00 23 51 2A 0 32 176 9 28 3 18 4 32 8 01 01 00 23 53 3A 0 41 6 187 8 29 9 25 9 53 7 01 01 00 23 53 4A 0 21 4 246 7 29 7 11 5 24 WET Sensor User Manual v1 4 Taking Readings 13 Calibration This section mainly describes the different soil calibrations that are used to convert the WET Sensor s dielectric readings into soil moisture measurements The WET sensor also needs soil parameter and temperature compensation values in order to derive its measurements of pore water conductivity And the soil moisture deficit calculation requires root depth and field capacity values Soil Calibrations The WET Sensor calculates soil moisture values using a simple mixing formula that relates water content to the measured permittivity of th
8. 4 18 20 23 Theory 15 30 U Units 11 21 34 Unpacking 6 W Water content 4 5 8 10 11 12 14 15 16 17 21 22 23 25 26 29 31 32 33 34 35 Water deficit 26 36 WET CLI 6 12 15 21 WET Sensor User Manual v1 4
9. However if you are taking measurements in heavy clay or sand or some other unusual medium you may want to calculate a value that is specific to that medium Warning the soil parameter should be left at the default value of 4 1 unless you have measured it for your soil Changing it will significantly affect the EC readings especially in dry soils We suggest you use the following procedure 1 Take a sample of soil 300ml and put this in a wide mouth bottle that has a sealable cap The quantity of soil is not critical but needs to be enough so that you insert the WET sensor fully and avoid edge effects i e 7Omm diameter x 75mm high 2 Add tap water of approximately twice the soil volume 600ml The conductivity of the tap water is not critical but the volume needs to be sufficient to saturate the soil and leave enough fluid above the soil to immerse the WET Sensor 3 Mix very thoroughly preferably by shaking the closed container for 10 minutes 4 Letthe soil settle for an hour WET Sensor User Manual v1 4 Taking Readings 17 5 Measure the permittivity and conductivity EC of the free water on top of the soil using the WET Sensor with the HH2 set to display W E T amp b ECb 6 Push the WET Sensor down into the soil and measure the bulk conductivity EC and permittivity 4 in the saturated soil below 7 The soil parameter E s 0 can then be calculated from the formula e x EC C
10. ID since each different probe requires loading a new calibration file number between 0 255 Enter the rooting depth of your plants Root Depth 0 9950mm This is mostly used for calculating water deficits see below Describes the depth of the sensor if it is being used to generate a depth profile 0 9950 Use for erasing all stored readings Soil Type Mineral The HH2 requires a soil calibration in order to Organic convert the soil permittivity reading into water Sand content The HH2 provides a choice of Ca 4 generalised calibrations Mineral aie canal Organic Sand Clay 5 spaces for user calibrations Custom1 5 Custom5 5 specialised calibrations for greenhouse OO growing media only available if WET CL has coir been purchased Min wool v These calibrations are explained in the Calibration section WET Sensor User Manual v1 4 Taking Readings 11 Soil Set Up This soil parameter appears in the equation from which the Pore Water Conductivity EC is calculated The default value is 4 1 but values Parameter between 1 0 9 0 can be used The Calibration section explains how to measure a value for the soil parameter and the Technical Reference section describes how the parameter is used in the EC calculation The field capacity of the soil can be set between 0 100 vol soil water content l The Technical Reference section describes how Capacity this is used in the calculation of the
11. S SC EC Temperature compensation The WET Sensor readings can be temperature compensated if required For some applications it is useful to Know the true electrical conductivity at the measurement temperature In that case uncompensated readings are required and this is the default for the WET Sensor However for most applications the electrical conductivity is a means of estimating the ionic content of the pore water and for those applications it is necessary to compare readings after they have been adjusted to a standard temperature Unfortunately this isn t straightforward because e 20 C and 25 C are both used as standards e The temperature compensation percentage depends on the particular mixture of ions present in the pore water For example E 1 5 Pas not known The values quoted are approximate for 25 c 18 Taking Readings WET Sensor User Manual v1 4 The choice of standard temperature and compensation will depend on what is commonly used for comparison purposes within your application Sensor Calibration We usually refer to the WET Sensor measuring dielectric properties but this is not strictly true The sensor itself really measures the capacitance C and resistance or its reciprocal conductance G of the material between the rods It infers the dielectric properties using a sensor calibration file lt WETxxyyy cal gt where yyy is the sensor serial number xx is batch numb
12. Sensor User Manual v1 4 Technical Reference 25 Calculating Water Deficit Water Deficit is the amount of irrigation water or rainfall mm that has to be added to a soil profile in order to bring it back up to field capacity The size of the water deficit will depend on the depth of the soil profile that it relates to usually taken as some function of the crop rooting depth For WET Sensors the water deficit D in mm is defined as Ong 0 where Rooting Depth in mm zwc Field Water Capacity of the soil spanned by the rooting depth d Water content of the soil as measured Pore water conductivity The electrical conductivity of the bulk soil o is a function of both soil water content and the pore water conductivity Op This is very similar to the relationship that has been found between the electrical permittivity of the bulk soil the permittivity of the pore water amp and e g Nyfors and Vainikainen 1989 Malicki et al 1994 found a high degree of linear correlation between values of o and amp for a broad range of soil types The following discussion proposes a theoretical basis for the relationship between op and amp and explains how this is used within the WET Sensor to derive readings of pore water conductivity Bulk Soil Conductivity v Pore Water Conductivity Consider the water that can be extracted from the pores of the soil matrix The permittivity and conductivity of the p
13. User Manual v1 4 Introduction to the WET Sensor 5 Unpacking Your consignment should have the following parts Part Salescode Description WET Sensor Sensor either fitted with 1 5m cable and 25 way D connector d or 9 way D connector p or 5m cable and bare wire termination w CD or 3 mee CD or floppy disk with floppy disk eh probe calibration file Marval IL mae Moisture Moisture Meter with Meter battery and connector cap Substrate no physical WET ST 1 Optional calibrations calibrations part factory WET GH 1_ for a number of installed into the common greenhouse WET CL 1 growing media Carrying case for WET Sensor HH2 and spare batteries 6 Introduction to the WET Sensor WET Sensor User Manual v1 4 Care and Maintenance CAUTION WET 2 is unsuitable for use in hard soils or substrates unless holes are pre formed Rough handling may cause irreparable damage to the pins Sensor care The WET Sensor is designed to be robust and trouble free in normal use but please observe the following sensible precautions e Look after the sensor rods Don t attempt to push the probe through stones or extremely hard soil If in doubt use an insertion tool to make pilot holes before inserting the WET Sensor e Do not pull the WET Sensor out of the soil by tugging on its cable e The WET Sensor is fully sealed and may be safely immersed in water but the interface connect
14. an write the following approximation for ZC CO mig a Jz L E SE 17 The permittivity and conductivity of the bulk soil will be denoted by the subscript b The complex permittivity of the bulk soil is proportional to both s and a function of d g 0 For dry soil there is no water to facilitate ionic conduction so the conductivity of the bulk soil op O However dry soil material is still polarisable so E 0 O and Gu appears as an offset to e WET Sensor User Manual v1 4 Technical Reference 27 By assuming that g takes into account the proportionality constant it is reasonable to postulate the following form for the complex permittivity of the bulk soil E E o 8 9 8 Note that e is a complex value and includes dielectric and ionic loss However since o 0 we may approximate Es 0 by its real part E0 With this and Eq 7 substituted in Eq 8 amp can be written as O E o eglO j 2 0 II 0 An electrical model for a dielectric material such as soil between two electrodes is a lossy capacitor We can calculate the admittance Y of this soil filled capacitor The admittance is the inverse of impedance Z and is a complex quantity which is proportional to the permittivity of the bulk soil and can be defined by Y M6 E K 10 where K is a geometry factor which is determined by the distance between the electrodes and their areas in contact with the soil Note tha
15. and temperature Press to store this reading and prepare for the next one 8 Care and Maintenance WET Sensor User Manual v1 4 Taking Readings Main stages required for taking and storing a reading with an HH2 Plug in Plug the WET Sensor into 25 way D connector socket at the bottom of the HH2 Power up If the LCD display is blank press the key to wake it up If necessary press key again until HH2 displays Set Press the Set key and then the key until the HH2 Device displays Press again then the key until the HH2 displays Press and then Esc Note If WET option is not displayed WET Sensor calibration file must be loaded into HH2 This requires the HH2Read program see instructions in HH2 user manual Check Press Set and then until the HH2 displays Press Set again then until the HH2 displays Press Set and the HH2 will display current memory and battery condition In this example HH2 indicates 11 of its memory has been used and 27 of the battery life is remaining WET WET Sensor User Manual v1 4 Taking Readings 9 User Manual v1 4 Taking Readings 9 your Soil Type and possibly Soil Set Up as well before achieving accurate readings Insert Push the WET Sensor into the soil or substrate If ground is hard or stony use an insertion tool to make guide holes first Options These are detailed in the next section However although default HH2 settings will enab
16. ateral variations in soil water content Dirksen C Dasberg S 1993 Improved calibration of time domain reflectometry for soil water content measurements Soil Science Society of America Journal 57 660 667 Hilhorst M A Balendonck J Kampers F W H 1993 A broad bandwidth mixed analog digital integrated circuit for the measurement of complex impedances IEEE Journal of Solid State Circuits 28 No 7 764 769 Hilhorst M A 1998 Dielectric characterisation of soil Doctoral Thesis ISBN 90 5485 810 9 Wageningen Agricultural University Wageningen The Netherlands Kaatze U Uhlendorf V 1981 The dielectric properties of water at microwave frequencies Zeitschrift f r Phys Chem Neue Folge Bd 126 151 165 Mualem Y Friedman S P 1991 Theoretical prediction of electrical conductivity in saturated and unsaturated soil Water Resources Research 27 2771 2777 Malicki M A Walczak R T Koch S Fluhler H 1994 Determining soil salinity from simultaneous readings of its electrical conductivity and permittivity using TDR Proceedings Symposium on TDR in Environmental Infrastructure and Mining Applications September 1994 held at Northwestern University Evanston Illinois USA Special Publication SP 19 94 US Department of Interior Bureau of Mines 328 336 Nyfors E Vainikainen P 1989 ndustrial microwave sensors Artech Hous Norwood USA Rhoades J D Shouse P J Alves W J Manteghi N
17. be found from a simultaneous measurement of and op independently of 0 Contact problems have only a minor effect on o measurements To facilitate calibration E s 0 4 1 can be used as an average In this case calibration of the sensor for o is not required Note the conversion formula in equation 16 gives the wrong answers in purely aqueous solutions unless the soil parameter is set to zero In order to overcome this the HH2 changes its conversion formula when the permittivity is very close to water and simply sets O Op 32 Technical Reference WET Sensor User Manual v1 4 References 10 11 12 13 Topp G C J L Davis A P Annan 1980 Electromagnetic determination of soil water content Water Resour Res 16 3 574 582 Whalley W R 1993 Considerations on the use of time domain reflectometry TDR for measuring soil moisture content Journal of Soil Sci 44 1 9 White Knight J H Zegelin S J and Topp G C 1994 Comments on Considerations on the use of time domain reflectometry TDR for measuring soil water content by W R Whalley Journal of Soil Sci 45 503 508 Roth C H Malicki M A and Plagge R 1992 Empirical evaluation of the relationship between soil dielectric constant and volumetric water content as the basis for calibrating soil moisture measurements Journal of Soil Sci 43 1 13 Knight J H 1992 Sensitivity of Time Domain Reflectometry measurements to l
18. d weigh it Wo Insert the WET Sensor into the dry soil 9 0 and measure the permittivity Zu Then b aler It will usually have a value between 1 0 and 2 5 Step 3 Calculate the volumetric water content 4 of the 7 m original sample 0 L Step 4 fe Jet Oi Then b It will usually have a value between 7 5 and 11 5 Example 1 In a sample of moist soil the WET Sensor gives a reading of 4 9 06 l e VE 3 01 This sample weighs 1 18kg and has a volume of 0 75 litres 2 After drying the sample of soil the WET Sensor gives an output of 9 2 56 From this we can calculate 4 amp bo 1 59 16 Taking Readings WET Sensor User Manual v1 4 3 The dry sample now weighs 1 05 kg so the volume of water in the moist sample was 1 18 1 05 0 13 litres Volumetric water content of the sample 0 0 13 0 75 0 173 m m 4 By substituting in equation 1 b4 8 19 Pore Water Conductivity The WET sensor calculates pore water conductivity using the relationship xEC The symbols and derivation EC oe of the formula are explained E s E ap 0 in the Reference section The WET sensor directly measures and EC and calculates E from the temperature The remaining parameter GE Is called the Soil Parameter and varies slightly for different soils Soil parameter The default value is suitable for a range of both organic and mineral agricultural soils
19. dard deviations for o at the ten 0 steps calculated according to Eq 16 are listed in the last two columns The seventh column lists the measured o of the pore water extract Comparison of the o values measured in the soil solution and the o values calculated from and oj justifies the model of Eq 16 The values found for E 8 at which o O are listed in the sixth column Table 1 Soil composition and validation results Soil Clay Silt Gand Organic Offset Conductivity of pore water matter Op Im m SE Measured Calculated D H Average Std devn Glass beads 7 6 100 90 10 Rockwool 4 1 300 310 10 Groesbeek 10 70 20 0 95 2 7 250 200 10 Wichmond 14 EN 55 4 3 1 9 100 110 5 Ferralsol A 63 26 11 0 4 4 80 50 6 Munnikenland A0 56 3 5 5 8 310 290 20 Attapulgite 100 0 0 0 3 1 130 130 10 WET Sensor User Manual v1 4 Technical Reference 31 Conclusions The relationship between simultaneously measured values of the real part of the permittivity 4 and the electrical conductivity of the bulk soil o measured at the same frequency is approximately linear Their measurements are both affected equally by the shape of the electrodes by the contact between electrodes and soil and by the soil composition In general this applies for any soil where the water content 0 gt 0 10 Due to the linear relationship between and oj the ionic conductivity of the pore water in the soil op can
20. der to simplify the appearance of the equations 24 Technical Reference WET Sensor User Manual v1 4 Measuring Soil Moisture Whalley ref 1 White Knight and Zeggelin ref 2 and Topp ref 3 have shown that there is a simple linear relationship between the complex refractive index which is equivalent to V and volumetric water content d of the form V a t a 0 HI This equation appears to work very well for most non magnetic soils and artificial growing media over a range of frequencies between 1MHz and 10GHz The following graph shows composite data for a number of different agricultural soils taken with the WET Sensor at 20MHz Refractive Index vs Water Content in various mineral soils y 10 20x 1 83 R 0 97 a GC Gel Groesbeek m Wichmond Munninkenland linear fit Ww x D Ki o v e D 0 100 0 200 0 300 0 400 Water content 8 mm You can see from this graph that the accuracy of the water content measurements would be improved by using a different calibration for each soil However the improvement would be small typically 2 or 3 so a generalised mineral calibration is appropriate for a good range of agricultural soils Warning This is not the case with clay soils and a soil specific calibration may improve the accuracy by gt 10 This is also true of organic soils because that label covers a huge range of soil types WET
21. e soil using the equation 0 Je al 1 Since the WET Sensor measures lt s directly the calibrations consist of pairs of coefficients b and bh Generalised Soil Calibrations Calibration bo by WET sensor generalised soil calibrations 100 80 60 40 WER organic D pure sand mineral clay soil water content 6 20 0 10 20 30 40 50 60 70 30 measured soil permittivity Lei 14 Taking Readings WET Sensor User Manual v1 4 Warning It is very important to use the correct soil calibration You can see from the graph above that a WET sensor reading of 40 would be interpreted as 40 water content using the generalised clay calibration but 60 using the organic or sand calibrations If in doubt you must generate a soil specific calibration see below WET CL Substrate Calibrations These calibrations have been developed by PPO in the Netherlands see Acknowledgements and are specific to a range of common greenhouse growing substrates EEN Ne Density Porosity Calibration Description 3 kg m mineral wool or glasswool WET sensor vertical 45 60 ee mineral wool WET sensor horizontal ES WET GH 1 Coir compost based on coir processed coconut fibre Peat based ZS PESE COMPE 75 250 55 80 may include clay or perlite Min g h soil average of a wide range of 4100 greenhouse soils SAE co ls potting composts including e roots Soil s
22. er which contains sets of Capacitance and conductance readings obtained when the sensor was calibrated in reference fluids with known dielectric properties Each WET Sensor requires its own individual calibration file C G T Sensor calibration Dielectric properties conversion lt WETxxyyy cal gt amp Temperature offset adjustment ET EC T Temperature compensation be f Display WET Sensor User Manual v1 4 Taking Readings 19 Recalibration Periodically you should test the accuracy of the WET Sensor by checking the conductivity and permittivity readings in a low conductivity aqueous solution The Pore Water Conductivity reading should be checked against a reading obtained with an accurate conductivity meter Remember to set the temperature compensation to be the same in the conductivity meter and the HH2 You can check the permittivity reading ciel in tap water at temperature T C by comparing the value displayed by the HH2 against the following approximate formula erf 87 48 0 365T If the WET Sensor readings are outside specification it should be returned to the supplier for recalibration Connection Table below details WET sensor cable connections for current and previous cable versions Red PowerVin 510 9VDC Not RS232 20 Taking Readings WET Sensor User Manual v1 4 Lo e SuOuEUuIede Y LA jenueyy Jost Josu s LIAM 7 02 1e Ajdde suoneoyioads Aoeinooe jU9 U04D J
23. eters and o is different when the water present is bound to the soil matrix rather than free water The model of Eq 16 cannot be used for the conductivity due to ions moving through the lattice of ionic crystals in a dry or almost dry soil the model is only valid for the free water in the matrix Thus SE is not the value for if 0 For sand the free water content corresponds to 0 gt 0 01 but for clay it can be WET Sensor User Manual v1 4 Technical Reference 29 0 gt 0 12 Dirksen and Dasberg 1993 As a rule of thumb the model applies for most normal soils and other substrates used for growing such as Rockwool if 8 gt 0 10 The design of the Wet Sensor In Eqs 14 and 15 only the term g 0 is affected significantly by the shape of the electrodes by the contact between the electrodes and soil and by the soil composition This term is eliminated in Eq 16 due to its ratiometric form and so measurements of pore water conductivity based on this equation are relatively insensitive to contact problems The WET Sensor exploits this technique by making simultaneous readings of and o within a relatively small sampling volume and at the same frequency The probe is built around an ASIC developed specifically for dielectric sensors at IMAG DLO This operates as a vector voltmeter to make precision measurements of and as shown in the following diagram Impedance input channel i Y Impeda
24. hart below You can get an idea of the size of this error by waggling the WET Sensor gently in the Soil WET Sensor User Manual v1 4 Pore Water Conductivity accuracy The accuracy of your pore conductivity readings will depend on Source of error Associated with Notes Soil type Very fine clay soils Can display unusual dielectric properties which reduce accuracy Magnetic soils Some problems in e g Ferralsols Artificial media Require custom soil parameter if reading at low water contents lt 30 Salinity Very saline soils See chart below Water content See chart below Temperature lonic conductivity See Temperature Compensation varies with above temperature Probe insertion Poor contact The Pore Water Conductivity between WET reading is less sensitive to probe Sensor and soil contact than the Water Content reading but good contact will always improves accuracy Reading range The following graph gives an indication of the range of pore water conductivity EC that can be accurately measured by the WET Sensor at different soil moisture levels Pore water conductivity accuracy 2100 accuracy typical 5 reading max errors 10 of reading 1800 or 10 mS m1 E accuracy typical 10 reading 1500 7 max errors 20 of reading 1200 F om L 600 e 300 Pore water conductivity ec mS m 0 oi 02 03 04 05 O06 07 08 09 Soil water content 6 mim WET Se
25. ir any goods where the products or goods in question have been discontinued or have become obsolete although Delta T will endeavour to remedy the buyer s problem Delta T shall not be liable to the buyer for any consequential loss damage or compensation whatsoever whether caused by the negligence of Delta T our employees or distributors or otherwise which arise from the supply of the goods and or services or their use or resale by the buyer Delta T shall not be liable to the buyer by reason of any delay or failure to perform our obligations in relation to the goods and or services if the delay or failure was due to any cause beyond the Delta T s reasonable control 38 Technical Support WET Sensor User Manual v1 4 Service and Spares Users in countries that have a Delta T Distributor or Technical Representative should contact them in the first instance Spare parts for our own instruments can be supplied from our works These can normally be despatched within a few working days of receiving an order Spare parts and accessories for sensors or other products not manufactured by Delta T may have to be obtained from our supplier and a certain amount of additional delay is inevitable No goods or equipment should be returned to Delta T without first obtaining the agreement of Delta T or our distributor On receipt at Delta T the goods will be inspected and the user informed of the likely cost and delay We normally expect
26. le you to take a reasonable reading both the water content and pore water conductivity readings depend on your choice of soil type and calibration values It s very likely that you will need to Set Read Press to take a sample There will be a few seconds delay while readings are taken results will appear on the screen like this In this example the Water Content has been calculated to be 25 6 Arrow keys can be used to scroll down and view other parameters Pore Water Conductivity ECp If probe has been inserted into a medium with a low water content HH2 will not be able to calculate pore water conductivity accurately and instead displays Temperature Press key to save reading or to discard it Off HH2 automatically goes to sleep after 1 minute but you can turn it off manually by pressing key until title screen is displayed and then once more to switch off Pressing the key will wake it into the same state as when it went to sleep 10 Taking Readings WET Sensor User Manual v1 4 Options This table lists options available on the HH2 which are particular to the WET Sensor for details of the other menu options refer to the HH2 user manual This is a single letter data label A Z A number that is stored with each reading for Sample identification purposes It automatically increments with each reading but can be set manually between 1 2000 Label not usually required with WET Sensors Device
27. mises the effects of probe contact and soil moisture on the readings Temperature is measured using a miniature sensor built into the central rod The WET Sensor is designed to be used with the HH2 Moisture Meter but can also be interfaced to control systems for fertigation control Advantages of the WET Sensor e Rapid measurements 5 seconds of all 3 parameters e In situ Pore Water Conductivity measurements e Easy insertion into growing media and soil e Calibrations available for many soils and growing media e Stable accurate readings from O to 80 water content 0 to 600 mS m 0 to 40 C e Lightweight ergonomic design 4 Introduction to the WET Sensor WET Sensor User Manual v1 4 How the WET Sensor works Measurement Principle d When you insert the WET Sensor into soil and take a reading which is applied to the central rod and produces a small electromagnetic field within the soil The water content electrical conductivity and composition of the soil surrounding the rods determines its dielectric properties The WET sensor detects these dielectric properties from their influence on the electromagnetic field and sends this information to the HH2 The HH2 calculates Soil Moisture using its calibration tables Water has a dielectric constant cd 81 compared to soil 4 and air 1 calculates the Pore Water Conductivity and displays the Temperature WET Sensor
28. nce selector Sine wave Signal s current source processing O Phase shifter 90 Fig 2 Schematic representation of the ASIC circuitry for measuring dielectric properties Validation of the Theory The model of Eq 16 was evaluated for five different soils glass beads of 0 2mm diameter and a slab of Rockwool The soils were samples from the Dirksen and Dasberg 1993 experiment Their compositions are listed in Table 1 The salinity of the soil samples was not changed The salinity of the Rockwool slab and the glass beads were adjusted to o 0 3 S m and Op 0 1 S m respectively using water NaCl solutions Sufficient water was left on top of the 30 Technical Reference WET Sensor User Manual v1 4 saturated samples to measure o of the soil solution The WET Sensor was used for the measurement of and oj and of op in the water left on top of the samples The measurement of o was checked using a laboratory 4 electrode conductivity meter at 1 kHz Each material was dried in ten steps by slowly extracting solution from an initially saturated and thoroughly mixed sample In this way 10 water contents between 0 0 10 and saturation were created The change in 8 was measured using a balance Since the salinity of the pore water was not allowed to change with O drying by evaporation was avoided The measured o values are listed in the eighth column of Table 1 The average values with their stan
29. nsor User Manual v1 4 Specifications 23 Technical Reference Dielectric properties When an electric field passes through a material such as soil some of the energy in the field is transmitted unchanged some is reflected some is stored and finally some is absorbed and converted into heat The extent to which each of these occurs within a particular material is determined by its dielectric properties These are quantified by a parameter called the relative electrical permittivity 6 of a material which characterises its response to the polarising effect of an applied electric field It is usually represented as a complex number 6 je 2 where the real part of the permittivity e represents the energy stored and the imaginary component represents the total energy absorption or loss Both values are frequency and temperature dependent For a static electric field the real part of the permittivity is often referred to as the dielectric constant The energy losses include dielectric loss 3 and loss by ionic conduction EC WE m _ aM 3 where EC is the specific ionic conductivity of the material and is the radian frequency in rad s The frequency in Hz of the applied electric field is f 2r The permittivity for free space is amp 8 854 101 F m Note in the remainder of this theory we ve used the symbol o instead of EC for the electrical conductivity in or
30. on TU uwz X Gp X GG BuIsnoH WWO9 pou 31u29 Ja oWeIP wwe x Duo g9 saqoid DAA 6 0 G SUOISIBA W Z LAM s zju w snse w scz Duunp ywor AjeoldA 9 07 01 0 aunjesodwe Puneiado Did 0 p je s 10 08uUU0D q em 6 G9d 0 p je s J0 09UU0D q AeM GZ 9d 0 pajees qO0ld JUSWUOJIAUA add GC payddns suonesqijed Josuas ENPIAIPU uonesqijeg Aouenbal4 uo1 2 s Hulmojjo ul ydeub eas auod yo AyAONpuoo Euren ayejnojeo OU UOI EIQIeD DIjINeds jlos 194e COD GG 0 910 p sn SI YDIUAA suopeJqIjeo los paljddns Uu G0 0 F G9 0 0 0 SUOEIGII2D 79 LIM UM ww G voor gozo d Ui 9EM Ween ainsiow pos ran OL oe 0 39 Auanonpuocd eono ying 101 eyep Fues HI WU G90 O 1 0 3 0 O 0 euou ecsl ma 8 Aua SUOI JEDIJID9dS Reading range and error sources Water Content accuracy The accuracy of your water content readings will depend on Error source Associated with Notes Soil calibration doesn t match actual soil type Calibration Soil type Very fine clay soils Magnetic soils Salinity Probe Poor contact insertion between WET Sensor and soil 22 Specifications Errors in mineral and sandy soil are usually lt 3 or 4 but can be 5 to 10 in clay and organic soils without a soil specific calibration Can display unusual dielectric properties which reduce accuracy Require soil specific calibrations Some problems e g in Ferralsols High readings See c
31. ore water will be denoted by the subscript p The imaginary part of the complex permittivity of the pore water is In soil science it is 26 Technical Reference WET Sensor User Manual v1 4 more practical to use the conductivity of the pore water op which can be defined as _ L n J O in 5 O WEE WE Ey 5 WE where Oj represents the ionic conductivity of the extracted pore water Dielectric losses are frequency dependent and have a maximum at the relaxation frequency The relaxation frequency of water is 17 GHz at 20 C Kaatze and Uhlendorf 1978 The operating frequency of the WET Sensor is 20 MHz and at that frequency Zus is negligible so Eq 5 can be reduced to O EE 6 Usually o is referred to as the EC Electrical Conductivity of the pore water lonic conduction is a function of temperature In the case of a NaCl water mixture the conductivity increases by 2 1 per C The values quoted for o are often corrected for temperature dependence to a temperature of 20 C or sometimes to 25 C This temperature correction depends on the ionic composition of the solution and is not applied automatically by the WET Sensor The complex permittivity of the pore water amp is equal to that of pure water The real part of the complex permittivity of the pore water 80 3 at 20 C with a temperature coefficient of about 0 37 per C Kaatze and Uhlendorf 1981 By analogy with Eq 3 we c
32. ors are not sealed and should be kept dry e The WET sensor can be buried to a depth of 2 metres e Rinse the WET Sensor in tap water and wipe off after use Warning you need to take reasonable precautions to protect the WET Sensor from physical damage to the rods and from static damage When not in use it is advisable to keep the sensor with the rods inserted into conductive foam or use the packing materials provided WET Sensor User Manual v1 4 Care and Maintenance 7 Quick Start If your WET Sensor has been supplied with an HH2 and you are confident that you know what you re doing the following instructions should get you started with minimum delay The section below Taking Readings gives a more detailed explanation Note that the HH2 must have the appropriate probe calibration CAL file loaded before any readings can be taken This file is supplied with your WET sensor on a CD or floppy disk it may need to be reloaded into your HH2 see HH2 User Manual if the HH2 battery has not been continuously maintained 1 2 Plug your WET Sensor into the HH2 Turn the HH2 on by pressing the key Press Set scroll down to Device press again and check that the device is set to WET Press again scroll down and select a suitable soil type Carefully push the probe into the soil Press to take a reading Press to scroll down through the measured parameters water content pore water conductivity
33. pecific Calibrations If the generalised or WET CL calibrations are not appropriate for your soil you will need to do a soil specific calibration The aim of this is to generate the coefficients bo b which can be entered into the HH2 s custom calibrations Ideally you should do this by taking an undisturbed core of a known volume of damp soil and measure its permittivity Ex and weight at intervals while carefully air drying it Use the measured damp weights dry weight and volume to calculate the water contents corresponding to the measured permittivities Finally graph refractive index de against water content fit a trendline to the data points and then b and b will be the offset and slope of that line see e g graph in Theory section WET Sensor User Manual v1 4 Taking Readings 15 However for most purposes a simple 2 point calibration is sufficient and we suggest you use the following protocol You will need e aWET Sensor e anHH2withits Display option set to WET ECb en e anon metallic oven proof container for a sample of 1 litre of soil e access to a temperature controlled oven or equivalent for drying the soil sample Step 1 Collect a sample of damp soil disturbing it as little as possible so that it is at the same density as in situ Insert the WET Sensor into the sample and measure the permittivity Zu Weigh the damp sample W and measure its volume L Step 2 Oven dry the sample an
34. rence 37 Technical Support Terms and Conditions of Sale Our Conditions of Sale ref COND 1 00 set out Delta T s legal obligations on these matters The following paragraphs summarise Delta T s position but reference should always be made to the exact terms of our Conditions of Sale which will prevail over the following explanation Delta T warrants that the goods will be free from defects arising out of the materials used or poor workmanship for a period of twelve months from the date of delivery Delta T shall be under no liability in respect of any defect arising from fair wear and tear and the warranty does not cover damage through misuse or inexpert servicing or other circumstances beyond our control If the buyer experiences problems with the goods they shall notify Delta T or Delta T s local distributor as soon as they become aware of such problem Delta T may rectify the problem by supplying replacement parts free of charge or by repairing the goods free of charge at Delta T s premises in the UK during the warranty period lf Delta T requires that goods under warranty be returned to them from overseas for repair Delta T shall not be liable for the cost of carriage or for customs clearance in respect of such goods However we much prefer to have such returns discussed with us in advance and we may at our discretion waive these charges Delta T shall not be liable to supply products free of charge or repa
35. t contact problems of the electrodes with the soil will be reflected in K The equivalent circuit for such a lossy capacitor is a loss free capacitor C with a conductor G in parallel C represents the energy storage capability of the soil and is related to Ex G represents the energy loss and is related to oy Y may be written in terms of C and G as Y G to 11 From Eq 10 and Eq 11 and with Eq 3 to Eq 9 in mind the real and imaginary parts of Y can be found G 0 g K 12 and C B Le et ik 13 28 Technical Reference WET Sensor User Manual v1 4 In terms of the measurable bulk quantities op and Ei e 7 8 9 14 and e TEEN oe 15 From Eq 14 and 15 the ionic conductivity of the pore water can be written as SE 16 be 01 0 Ze The model of Eq 16 describes the relationship between o of the pore water the water that can be extracted from the soil and the values and o as measured in the bulk soil using a dielectric sensor The offset SE can be calculated from the E and values measured at two arbitrary free water content values Munnikenland 26 80 5 8 R 0 992 Ferralsol ei 160 4 o 4 4 R 0 991 Op S m Op S m Fig 1 Examples of the relationship between and op showing the offset E s 0 for two different soils The relationship between the bulk soil parameters and op and the corresponding pore water param
36. to complete repairs within a few working days of receiving the equipment However if the equipment has to be forwarded to our original supplier for specialist repairs or recalibration additional delays of a few weeks may be expected Technical Support Technical Support is available on Delta T products and systems Users in countries that have a Delta T Distributor or Technical Representative should contact them in the first instance Technical Support questions received by Delta T will be handled by our Tech Support team Your initial enquiry will be acknowledged immediately with a T number and an estimate of time for a detailed reply Make sure to quote our T number subsequently so that we can easily trace any earlier correspondence In your enquiry always quote instrument serial numbers software version numbers and the approximate date and source of purchase where these are relevant Contact details Tech Support Team Delta T Devices Ltd 130 Low Road Burwell Cambridge CB25 OEJ U K email tech support delta t co uk Web site www delta t co uk Tel 44 0 1638 742922 Fax 44 0 1638 743155 WET Sensor User Manual v1 4 Technical Support 39 Acknowledgements These sensors have been developed in co operation with GREENHOUSE TECHNOLOGY WAGENINGEN EGS Institute of Agricultural and Environmental Engineering IMAG BV P O Box 43 6700 AA Wageningen the Netherlands Web site www wur nl Designers
37. ty of soil type only and typically varies between 0 1 mm for sandy soils up to about 0 45m m for clay soils EE all water no soil 100 Through drainage D SV ee eS Saturation Es waterlogged DEE EE Field Capacity Proportional to Deficit plant water use e es Wilting point plant stressed oven dry 0 Time _ Wilting Point WP is the moisture content at which a particular crop is unable to extract any more water Conventionally this is taken to correspond to a matric potential of 1500kPa but it s really much more variable than that suggests It s a property of soil type and crop type and can vary between about 0 04 mi mt for sandy soils to 0 22 m m for clay soils Dry is zero moisture content Available Water Capacity is the difference between Field Capacity and Wilting Point Water Deficit is the amount of irrigation water or rainfall mm that has to be added to a soil profile in order to bring it back up to field capacity 36 Technical Reference WET Sensor User Manual v1 4 Interfacing to the WET Sensor If you want to use the WET sensor for irrigation or fertigation control there is a separate manual detailing connections and data handling requirements for WET1 WET sensor Application note Wi en March 30 2001 fageningen Jos Balendonck Max Hilhorst IMAG B V Report Number 2001 07 WET Sensor User Manual v1 4 Technical Refe
38. uctivity of the soil and is a function of pore water conductivity soil particle conductivity soil moisture content and soil composition Saturation Extract is the solution extracted from a soil at its saturation water content Because it is not easy to determine the saturation water content the saturation extract is usually approximated by adding excess water e g 5 litres of water to a 1 litre sample of soil and then adjusting readings from the resulting extract appropriately Soil Extract is the solution separated from a soil whether saturated or unsaturated by filtration suction or centrifuge All these techniques introduce errors because the extracted solutions differ significantly from the pore water 34 Technical Reference WET Sensor User Manual v1 4 Soil Salinity is the concentration of all soluble salts within a soil It usually requires laboratory analysis to determine soil salinity directly so it is conventionally represented by the electrical conductivity of the saturation extract ECseg Soil salinity is partitioned into the following descriptive categories _non saline 0 200 mS m _ _slightly saline 200 400 1 m m f moderately saline 400 200 mS m _strongly saline 800 1900 1 m m extremely saline gt 1600 m m Soil Water Content is defined either in relation to the volume of soil V 8 water mm A sample or in relation to the mass of the dry soil m water 1 Bee ae
39. web www delta t co uk Contents Introduction to the WET Sensor E Description 4 How the WET Sensor works 5 Unpacking 6 Care and Maintenance 7 Sensor care 7 Quick Start 8 Taking Readings 9 Options 11 Calibration 14 Soil Calibrations 14 Pore Water Conductivity 17 Recalibration 20 Connection 20 Specifications 21 Reading range and error sources 22 Technical Reference 24 Dielectric properties 24 Measuring Soil Moisture 25 Calculating Water Deficit 26 Pore water conductivity 26 References 33 Definitions of Terms 34 Interfacing to the WET Sensor 37 Technical Support 38 Acknowledgements 40 Index 41 WET Sensor User Manual v1 4 Introduction to the WET Sensor 3 Introduction to the WET Sensor Description The WET Sensor is a multi parameter sensor for use in soils composts and other artificial growing media It measures the dielectric properties of the soil and calculates e Water Content Electrical Conductivity Temperature The sensor converts the measured dielectric properties into Water Content over the full range 0 80 using calibration tables Generalised calibrations are provided for most common soil types and specialised calibrations are available as separate cost options for a number of artificial substrates The WET Sensor also calculates Pore Water Conductivity the Electrical Conductivity of the water within the pores of the soil EC Its calculation is based on a unique formula that mini
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